This invention relates to Internet telephony, and more specifically, to an improved technique of accurately transmitting dual tone multi-frequency (DTMF) tones reliably and with minimum overhead over a data network such as the Internet.
Internet telephony is a rapidly growing field and is expected to continue growing. Internet telephony involves the transmission of telephone traffic over a data network. The use of a data network, and the transmission of the telephone call using packet switching techniques rather than circuit switching techniques as in conventional telephony, allows a variety of value added services to be implemented. Additionally, use of the Internet results in a much less costly telephone call, since data traffic over the Internet is virtually free.
Typically, two audio terminals communicate by each establishing a local circuit switched connection to a local Internet gateway. The gateways then communicate with each other over the Internet using the standardized Internet protocol (IP) and well-known packet switching techniques. Thus, a long distance call may be implemented using two local telephone calls and a long distance Internet connection.
The transmission of voice and other audio traffic over the Internet is done using digitization and encoding techniques. Specifically, the International Telecommunications Unit (ITU) has defined a G.723 standard for transmission of voice over the Internet. The standard specifies the particular mathematical encoding algorithm that is utilized in order to compress and encode the digital voice for packet transmission over the Internet.
Unfortunately, the particular mathematical encoding algorithm does not accurately and efficiently encode signals which represent DTMF tones. Moreover, since a DTMF tone is simply an analog signal representative of a digit pressed, there is no need to transmit the entire analog signal over the digital data network. Rather, the DTMF digit may be transmitted as digital data over the Internet.
The ITU standard for Internet telephony specifies that the digital data is to be transmitted out of band, i.e., as a digital signal separate from the encoded audio signal. There appears to be no efficient standard method for transmitting DTMF tones and for minimizing the latency, overhead and errors associated therewith.
In view of the above, there exists a need in the art for an improved and efficient method of transmitting DTMF tones from an Internet telephony gateway, over the Internet to a receiving Internet telephony gateway. Additionally, the technique should minimize the additional overhead associated with transmission of the DTMF tones.
The above and other problems in the prior art are overcome and a technical advance is achieved in accordance with the present invention which relates to a DTMF detector installed in parallel with a voice coder running at the Internet telephony transmitting gateway. In accordance with the invention, a very fast algorithm is used (e.g., 10 milliseconds detecting time) to determine whether or not a tone is suspected in the data stream being transmitted. If such a tone is suspected, then transmission is held up for several frames until a tone is confirmed. If, prior to the expiration of the several frame period, a xe2x80x9ckillxe2x80x9d signal is received, then it is determined that the audio stream did not include a DTMF.
If the foregoing decision indicates that a DTMF tone is present, the actual digitized audio reflecting the DTMF signal is discarded, and a digital signal is sent, out of band, indicating to a receiving gateway that the DTMF tone has been depressed. If, on the other hand, the algorithm at the transmitting gateway indicates that a DTMF tone has not been depressed, and that the suspected DTMF tone was actually a small portion of the audio signal that was simulating a DTMF tone, then the audio signal suspected of being a tone is transmitted. Any delay introduced to the audio signal by the suspecting of tone and the later finding out that the tone was not actually present in the audio signal is compensated for by deleting later arriving portions of the audio signal which are determined to contain silence.
In another embodiment, the DTMF detector uses as its input the LPC filter present in the voice coder. This has two advantages. First, since the LPC encoding is being done as part of normal speech encoding, no additional overhead is introduced. Second, the FFT utilized to detect the DTMF frequencies operates on the LPC coefficients, rather than on raw digital data. Therefore, the DTMF tones can be detected with a relatively short FFT.